This invention relates to apparatus for filtering water containing radioactive substances, for example radioactive waste water produced in a nuclear electric power generating plant, or primary water of a boiling water type nuclear reactor.
As filtering apparatus of radioactive waste liquid produced in a nuclear electric power generating plant, it has been used a precise filtering device utilizing a filtering aid. But as ion exchange resin has been used as the filtering aid, a large quantity of used resin is produced thereby increasing the quantity of radioactive waste to be discarded. Since radioactive waste is dangerous, it has been necessary to sink it in deep seas or to store it in underground storage.
Recently, improved filtering apparatae which do not produce any secondary waste and can obtain treated water of better quality have been developed. Examples are an ultrafilter (hereinafter termed UF), and a filter comprising a flat film having numerous pores each having a diameter of 1 micron. However, since the ultrafilter must circulate a large quantity of water, not only the construction of the circulation system is complicated but also the cost of installation increases, whereas the flat film has a large tendency of clogging the minute pores with solid substances. In addition, it requires a large installation area.
In a nuclear electric power generating plant too, the same type of filters are used in a condensate feed system as well as primary water feed system for a nuclear reactor.
The particle size distribution of insoluble impurities (CRUD) contained in radioactive waste liquid and condensate or primary water fed into a boiling water type nuclear reactor is shown in FIG. 1. Although the particle size distribution varies more or less depending upon the type of the liquid or water to be filtered, a substantive portion of the CRUD has a particle size of about 1 micron, and the percentage of CRUD having a particle size of 0.1 micron or less is very small.
Consequently, in a UF having pores having a diameter much smaller than 0.1 micron, substantially no solid particles enter into the pores so that there is no tendency of clogging the pores with the solid particles because trapped solid particles can readily be removed by backwashing. Since in the UF, the pore size is small, the quantity of water that can be filtered is small which not only increases filtering area but also requires a high filtering pressure which also increases the cost of installation.
In a flat film type filter having a pore size of about 1 micron, since the waste liquid contains relatively large quantity of solid particles having a diameter of less than 1 micron, the tendency of clogging the pores increases whereby the film must be renewed frequently.
Recently, hollow fibers made of polyvinyl alcohol or copolymers thereof with styrene, etc., and having a sponge-like mesh structure were developed as disclosed in Japanese Laid-Open Patent Specification No. 73390/1980. The hollow fiber disclosed therein has a diameter of 0.5-5 mm and contains numerous pores interconnected to form a complicated mesh structure. Each pore has a diameter of 0.005-1.0 micron, preferably 0.01-0.5 micron. This polyvinyl alcohol hollow fiber has an excellent water permeability, and high acid resistant property. Usually, 1,000-5,000 fibers are bundled in a U-shape and water containing iron is passed through the U-shaped bundle of fibers. Then iron is trapped in the pores and high purity water suitable for use in boilers or preparation of foodstuffs or drinks can be obtained on the outside of the fibers.
But these polyvinyl alcohol fibers are solid having low mechanical strength so that they fracture or damage when subjected to a relatively small force or shock. Furthermore, clogged fibers cannot be regenerated with backwashing unless a special chemical agent is used. Presence of a chemical agent in nuclear reactor water, especially in a boiling water type reactor, is not permissible. In a nuclear power plant, water utilized in the plant is desired to have much higher purity than other applications. In other words, the concentration of solids in water should be extremely small.
When filtering water used in a nuclear power plant with hollow fibers having a sponge like mesh structure of the type described above, not only dense films of solids are formed on the surfaces of the fibers but also a large quantity of the solids flows into the pores thus clogging the same. In other industrial applications, since the concentration of the solids is high, they aggregate into large particles which close the openings of the pores, whereby even when solids of small particles are present in the water to be filtered, such small particles are prevented from entering into the pores, thus preventing clogging thereof. For this reason, in the case of a nuclear power plant, the pore diameter must be selected carefully for preventing clogging of the pores.